The present invention relates to a pressing method for dewatering a fibre web, such as a paper or cardboard web, in a paper machine press section including a press nip defined by two press rolls and wherein the web to be dewatered is conducted through the press nip either being carried by a single fabric or interposed between two fabrics.
The invention further relates to an apparatus for carrying out the method of the invention, the apparatus comprising two press rolls defining a press nip, one of the press rolls preferably being a smooth-surfaced roll while the other preferably being a recessed-surface roll, and wherein one or more press fabrics are conducted through the nip.
The most common method of dewatering fibre webs and, in particular, paper or cardboard webs, comprises conducting the web through a press nip defined by two rolls in opposed relationship with each other. Preferably, one or two press fabrics are conducted through the dewatering press nip in order to carry away the water pressed from the web and to also function to transport the web from the press nip.
As the production rates of paper machines increase, the dewatering of the web in the press section has become a major limiting factor with regard to production rates due to the fact that the press nips defined by a pair of rolls have a relatively short region of action and, therefore, the time spent by the web in such press nips is relatively short at higher web speeds. A certain minimum time, however, is required in view of the flow resistance presented by the fibre structure of the web for the water to escape from the web and enter into the recessed surface of the press roll or rolls or the press fabric. For this reason, a plurality of successive press nips have been utilized in the past. For example, so-called compact press sections, such as the so-called "Sym-Press" press section manufactured by Valmet Oy of Finland, or several separate successive press nips have been used in conventional press sections. However, press sections incorporating several nip presses have large space requirements, especially where separate successive press nips are used. A compact press section of the type mentioned above presents difficulties with respect to the optimal placement of their various components as well as in their operation, e.g., with respect to carrying off paper broke. Moreover, expensive suction rolls are commonly employed in nip presses, such suction rolls having significant energy requirements for producing the vacuum and since a perforated shell is a necessity for such suction rolls, problems of mechanical strength also arise in connection with their use.
Further, a web can only tolerate a certain maximum nip pressure so that increases in dewatering through corresponding increases in nip pressures are limited by the pressures which can be tolerated by the web.
Attempts to lengthen the pressing zone of the press nips through the use of larger diameter rolls and/or soft press fabrics are also limited for economic reasons, among others.
However, in view of the various dewatering considerations discussed above, and for other reasons, so-called long nip presses have recently been designed. For example, such long nip presses are disclosed in U.S. Pat. Nos. 3,808,092; 3,808,096; 3,840,429; 3,970,515; 4,201,624 and 4,229,253, and British patent application No. 2057027.
Further regarding the state of the art, reference is made to Finnish patent application No. 3554/72 and U.S. Pat. No. 3,783,097. A paper machine press arrangement is disclosed in said Finnish patent application for dewatering a paper web wherein a long pressing zone is provided by means of appropriately tensioning flexible belts. This arrangement, however, has the drawback that the mechanical strength of the press belts and their associated guide rolls impose a limit on the pressure which can be exerted on the web and, therefore, on the web dewatering which can be accomplished. A long nip press is disclosed in the above-mentioned U.S. patent wherein a plurality of successive pressure shoes are urged towards a belt and opposing press roll. However, this arrangement has the drawback that the friction between the pressure shoes and the belt results in the consumption of large amounts of energy. Additionally, the belt and pressure shoes are subject to considerable attrition as a result of friction between them.
A press section incorporating a long nip press is disclosed in the above-cited U.S. Pat. No. 3,840,429 wherein the web being pressed runs while interposed betwcen two felts rectilinearly through the press zone defined by two opposed press shoes. Pressure is produced in the press zone by means of a pressurized fluid. Bands are provided within the loops of the felts which confine the press zones and transmit the pressure of the pressurized fluid to the web. However, this long nip press is not entirely satisfactory since sealing problems are encountered in the press zone. Another drawback of this arrangement is that the web is immediately subjected to the entire, and necessarily relatively high, pressure in the nip. However, since the web has a relatively low dry matter content, it cannot tolerate pressures exceeding a certain maximum without breaking For this reason, the nip pressure must be maintained at a relatively low level. Moreover, it is generally not advisable to subject a web to a high nip pressure which rises abruptly at the very initiation of the pressing operation.